فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri - Paris Observatory

The region around a celestial body in which the magnetic field of the body dominates
the external magnetic field. Each planet with a magnetic field (Earth,
Jupiter, Saturn, Uranus, and Neptune) has a magnetopause.
The Earth's magnetosphere is a dynamic system
that responds to solar variations.
It prevents most of the charged particles carried in the
→ solar wind, from hitting the Earth.
Since the solar wind is → supersonic,
a → bow shock
is formed
on the sunward side of the magnetosphere.
The solar wind ahead is deflected at a boundary called → magnetopause.
The region between
the bow shock and the magnetopause is
called the → magnetosheath.
As the solar wind sweeps past the Earth, the terrestrial magnetic field
lines are stretched out toward the
night side to form a → magnetotail.

An analysis technique applied to some atomic nuclei that have the property
to behave as small magnets and respond to the application of a magnetic
field by absorbing or emitting electromagnetic radiation. When nuclei
which have a magnetic moment (such as 1H, 13C, 29Si,
or 31P) are submitted
to a constant magnetic field and at the same time to a radio-frequency
alternating magnetic field, the nuclear magnetic moment is excited to
higher energy states if the alternating field has the specific resonance
frequency. This technique is especially used in spectroscopic studies of
molecular structure and in particular provides valuable information in medicine that can
be used to deduce the structure of organic compounds.

In the context of solar physics, a → magnetic field line
when it crosses the solar surface only once, i.e., when it goes from surface to infinity.
This is the case at a sufficiently large scale in → coronal holes.
This is mostly not the case in → active regions.

The property of a substance that possesses a → magnetic permeability
greater than that of a vacuum but significantly less than that exhibited by
→ ferromagnetism.
In the absence of an external magnetic field the atomic
→ magnetic moments of the substance
are randomly oriented and thus cancel each other out with no net total magnetic moment.
Moreover the coupling between neighboring moments is weak.
However, when a magnetic field is applied magnetic moments align with the direction of the
field and so the magnetic moments add together.
Therefore paramagnetic substances
affect external fields in a positive way, by attraction to the field
resulting in a local increase in the magnetic field.
The → magnetization vanishes when the field is removed.

A piece of magnetic material which, having been
→ magnetized, retains a substantial proportion
of its → magnetization indefinitely.
In permanent magnets the magnetic field is
generated by the internal structure of the material itself.
Atoms and crystals constituting materials are made up of electrons and atomic nuclei.
Both the nucleus and the electrons themselves act like little
magnets. There is
also a magnetic field generated by the orbits of the
electrons as they move about the nucleus. So the magnetic fields of
permanent magnets are the sums of the nuclear spins, the electron
spins and the orbits of the electrons themselves. In many materials,
the magnetic fields are pointing in all sorts of random directions
and cancel each other out and there is no permanent magnetism. But in
certain materials, called → ferromagnets,
all the spins and the orbits
of the electrons will line up, causing the materials to become
magnetic.
Many permanent magnets are created by exposing the magnetic material
to a very strong external magnetic field. Once the external magnetic
field is removed, the treated magnetic material is now converted
into a permanent magnet.
Overheating a permanent magnet
causes the magnet's atoms to vibrate violently and disrupt the
alignment of the atomic domains and their dipoles. Once cooled, the
domains will not realign as before on their own and will
structurally become a temporary magnet
(MagLab Dictionary).

A dense zone of magnetized → plasma surrounding a
→ pulsar. The magnetosphere, lying between the surface of
the → neutron star and the
→ light cylinder,
corotates with the pulsar like a rigid body under the effect of
strong magnetic field. The magnetosphere's thickness is determined by the constraint
that the corotation velocity of its upper surface should not exceed the
→ speed of light.

The Sun's magnetic field which is probably created by the
→ differential rotation of the Sun together with
the movement of charged particles in the → convective zone.
Understanding how the solar magnetic field comes about is the fundamental problem of
Solar Physics. The solar magnetic field is responsible for all solar magnetic phenomena,
such as → sunspots, → solar flares,
→ coronal mass ejections, and the
→ solar wind. The solar magnetic fields
are observed from the → Zeeman broadening of spectral lines,
→ polarization effects on radio emission, and from the
channeling of charged particles into visible → coronal streamers.
The strength of Sun's average magnetic field is 1 → gauss
(twice the average field on the surface of Earth, around 0.5 gauss),
and can be as strong as 4,000 Gauss in the neighborhood of a large sunspot.

The magnetic moment associated with the → spin angular momentum
of a charged particle. The direction of the magnetic moment is opposite to the direction
of the angular momentum. The magnitude of the magnetic moment is given by:
μ = -g(q / 2m)J, where q is the charge, m is the mass,
and J the angular momentum. The parameter g is a characteristic of the
state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some
other number in between for a complicated system like an atom. The quantity
in the parenthesis for the electron is the → Bohr magneton.
The electron spin magnetic moment is important in the → spin-orbit
interaction which splits atomic energy levels and gives rise to
→ fine structure in the spectra of atoms.
It is also a factor in the interaction of atom with external fields,
→ Zeeman effect.

The → magnetic field associated with a
star. Magnetic fields are common among stars of solar and lower
masses. So far definitive detections of fields in stars with masses
~1.5 Msun have, for the most part, been made for objects
having anomalous chemical abundances (e.g., the
→ chemically peculiar A and B stars).
Recently, however, observations of cyclic variability in the
properties of → stellar winds
from luminous → OB stars
have been interpreted as evidence for the presence of
large-scale magnetic fields in the surface layers and atmospheres
of these objects (→ magnetic massive star).
These inferences have been bolstered by
the unambiguous measurement of a weak (~ 360 G) field in the
chemically normal B1 IIIe star → Beta Cephei.
These results suggest
that magnetic fields of moderate strength might be more
prevalent among → hot stars
than had previously been thought.
At the present time, the origin of magnetism in massive
stars is not well understood.
If the magnetic field of a hot star is produced by
→ dynamo effect
in the → convective core,
then a mechanism for transporting
the field to the stellar surface must be identified. The
finite electrical conductivity of the envelope leads to the outward
diffusion of any fields contained therein, but only over
an extended period of time. Estimates indicate that for stars
more massive than a few solar masses, the resistive diffusion
time across the radiative interior exceeds the
→ main sequence lifetime. Another possibility
is that dynamo fields are advected from the core to the surface
by rotation-induced → meridional circulation
(MacGregor & Cassinelli, 2002, astro-ph/0212224).

A magnetic field which is generated in a → plasma
inside a → toroid,
as in a → tokamak, by the electric current which
spirals around the toroid. Toroidal field has no radial component.
→ poloidal magnetic field.